Mixtures of particulate coal and petroleum oil have been employed in the past as fuel for boilers, steel mill blast furnaces and the like; and much of the early development work on these mixtures involved schemes for utilizing substandard coal and fines. Modernly, the desire for more efficient use of domestic energy reserves has led to renewed efforts to optimize the utility of slurries of pulverized coal and petroleum liquids for fuel purposes.
It is well known that simple mixtures of finely crushed coal and petroleum oil ordinarily exhibit undesirable particle settling, agglomeration and compaction; and the hard sediments which ultimately result tend to resist redispersion and consequently render the mixture unusable. Continuous mechanical agitation has been practiced heretofore as a means of preventing coal sedimentation. However, sedimentation and compaction can occur, even in agitated systems, wherever quiescent regions occur, such as adjacent tank corners and near flow-control valves. Reducing the coal to micron size in order to promote suspension has likewise been proposed, but this approach has proved to be unacceptably expensive.
Various chemical additives have also been developed in the past for stabilizing a suspension of coal macroparticles in a petroleum liquid co-fuel; and many of these additives act through thixotropicity or through the creation of micelles in the fluid medium. Regardless of such chemical advances in resisting sedimentation, coal and oil mixtures continue to exhibit various commercial deficiencies, notably among them being the propensity to produce air pollution upon combustion.